LONDON Analog ICs and discrete devices specialist Zetex Semiconductors is claiming a major breakthrough in Low Noise Block (LNB) design with a 4:2 IF switch that integrates gain and control function that it says significantly improves IF signal reliability, eliminates false switching and reduces circuit complexity.
"We have already sampled the device to some makers of twin and quad LNBs and expect the part to be in products being shipped early next year that will have a far reaching impact on satellite TV distribution networks," Richard Robinson, marketing manager of the DBS division at Zetex told EE Times Europe .
Robinson said the device achieves 30dB channel isolation up to frequencies of 2GHz, "which is significantly better than anything achieved so far", and is typically 4dB better than the target recommended by [satellite operator] Astra for LNBs in Europe.
Zetex says that by adding 19dB of gain, the device, dubbed the ZXHF5000, also avoids the 7dB insertion loss normally prevalent in traditional passive switch solutions. "Until now, achieving the combination of high isolation and positive gain has proven too difficult to overcome."
Geoff Stokes, DBS Systems Engineer at Zetex commented: "Attaining this level of isolation is simply not an easy task. We are not doing any frequency conversion within the IC and so the circuit layout and track routing of the silicon design was crucial to the chip performance, as was pin-out and package selection.
"European satellite operators are typically recommending a minimum isolation figure of 26dB, and it has been noted that this key design metric has been gradually reducing in some LNBs. Poor isolation means that LNB output signals are closer together in frequency and in a weak signal area this has an adverse effect on signal reliability and STB performance."
Size, weight and silicon cost are all about a third less than for current generation twin output LNBs, said Robinson. "These are significant improvements, especially as the satellite dish LNB market, like the set top box business, is extremely competitive."
Robinson notes the part was designed for operation at 3.3V, which will help with power efficiency, increasingly an important issue for satellite operators.
To save power the chip shuts down any unused output cable driver stages and any unused input stages while maintaining impedance matching with pre-amplifiers or mixers.
Achieving the gain characteristics presented a major design challenge. While the traditional demand is for LNBs to handle a bandwidth of 950 to 2150MHz, an extended band of 200 to 2150MHz is now being advocated, to support more TV channels and the introduction of HDTV. The device was designed to provide a flat 19dB gain across the band with a slight increase in gain (2dB) between 1500MHz to 2500MHz to help with downfeed cable losses.
The RF inputs were also designed such that they can be reprogrammed to be in reverse order, by applying a logic control signal, thus increasing flexibility and to help minimize PCB area. Through this novel mirroring function, two devices can be mounted back-to-back on a double-sided PCB such that the two sets of RF inputs are aligned. The 4x4 IF switch created therefore helps avoid complex routing of RF tracks.